A knowledge of the intermolecular force field makes possible the theoretical calculation and prediction of all properties of a substance which depend on this field. A practical simplification of the general intermolecular force field is to express the nonbonded energy as a pairwise sum of atomic contributions. This procedure has been very successful in the quantitative calculation of the crystal structures, heats of sublimation, thermal expansions, vibrational frequences, and elastic constants of hydrocarbons. In certain cases the atom-atom nonbonded potential method also applies to interactions between different parts of the same molecule, e.g., polymers and biopolymers. Recent developments indicate that coulombic interactions, previously neglected by most researchers, are highly significant even in hydrocarbons. These coulombic interactions will become even more signficant in the force fields of molecules more polar than hydrocarbons. The hydrogen bond is a transitional case where strong coulombic interaction merge toward chemical valence forces. A difficulty in the application of the atom-atom potential method has been the lack of availability of quantitative nonbonded potential parameters. Much more accurate hydrocarbon potentials have recently become available. The objective of this proposal is to extend the quantitative derivation of nonbonded potential parameters beyond simple hydrocarbon molecules to substances containing nitrogen, oxygen, halogens, and/or hydrogen bonds. The method is to find the best adjustable parameters for model atom-atom potentials which will reproduce selected known intermoleculr structures. The optimum parameters are those which most closely predict all the pertinent intermolecular properties of the basis experimental data. An important goal is to achieve force fields which will be transferable and therefore useful in dealing with new substances and with unknown intermolecular situations.